eBay Error Code 21916271: Specified SKU is in use

Long Message: When specifying SKU for InventoryTrackingMethod, the SKU must be unique within your active and scheduled listings

Resolution:

If you see this error returned by eBay, that would indicate there is already an existing listing with that ebayskuon the channel. You would be able to check this when you go directly on eBay and search for the item's ebaysku value (generated based on the item's guid) in the channel inventory, which is basically eBay's Custom Label and if the listing is still live and something has caused a disconnect, you still would be able to get them back in sync and you can follow the steps outlined in this guide.